1. Field of the Invention
This invention relates to probe cards used in the precision testing of microelectronics. More particularly, the invention relates to ceramic probe cards and methods of treating such cards to minimize leakage current between electrical conductors on the surface of the card.
2. Description of Related Art
Probe cards are used in the testing of micro-electronic devices, particularly for in line parametric testing during the manufacture of such devices, to monitor diffusion depths and other critical parameters. The probe card typically provides multiple channels, defined by corresponding conductors, each channel carrying a signal between test instrumentation and a probe that contacts a point on the device under test. The tests may require that current levels be measured to an accuracy on the order of ten femtoamps (10.sup.-14 amps), or less.
To make such accurate measurements, current leakage on the probe card must be reduced to correspondingly low levels, and a number of techniques for leakage current reduction are known. A basic technique is to construct the probe card of a material that inherently has good electrical insulating properties. Ceramic is such a material, however, leakage current between adjacent electrical conductors on a ceramic card having no specialized shielding may be as high as 10 picoamps (10.sup.-11 amps). This is several orders of magnitude too high. Thus, despite the excellent insulating properties of ceramic, additional steps must be taken to minimize leakage current.
One such technique that provides significant reduction in leakage current is the use of active guard technology in which conductors on the surface of the ceramic probe card are surrounded with active guard shielding. The conductors being shielded are referred to as the "force" conductors. The conductors doing the shielding are referred to as the "guard" conductors.
Unlike conventional shielding where the shield conductors are grounded, in active guard technology the guard conductors are held at a voltage that is identical to the voltage on the force conductors that they are guarding. This arrangement produces an equipotential surface in the vicinity of each force conductor which inhibits the flow of leakage current. Using this technology, leakage may be reduced to levels on the order of 0.1 picoamps (10.sup.-13 amps), however, for some tests that it is desirable to make, this is still too high.
The present invention addresses the leakage current problem on prior art ceramic probe cards having active guard shielding and results from a careful investigation of the source of the remaining leakage. Without intending to be limited by any theory of operation of the present invention, it appears that much of the remaining leakage current arises from atmospheric humidity that produces a current path on the surface of the ceramic card. The present invention addresses this problem by providing a ceramic probe card which has a hydrophobic moisture repelling material bonded to the surface of the ceramic between the electrical conductors.
Problems can arise, however, whenever the ceramic surface interface is coated with an additional material. Contaminants on a prior art ceramic probe card surface, for example, can increase leakage current drastically. Consequently, any intentionally added hydrophobic coating material must be an excellent electrical insulator and/or be capable of being applied in extremely thin layers to avoid producing an additional conductive path. Also, the coating must be capable of surviving subsequent exposure to hot solvents, soldering resins and heat to which the ceramic surface may be subjected, but must not interfere with soldering to the conductors. Preferably the additional material should strongly adhere to the ceramic surface and yet be easily removed from the conductors. If the additional material lacks this property it must be very carefully applied only to the exposed ceramic surface between closely spaced conductors, leading to a very expensive and difficult application step.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a ceramic electrical probe card in which leakage currents have been reduced to very low levels.
It is yet another object of the present invention to provide a ceramic probe card in which the leakage has been reduced through the use of an easily applied hydrophobic material.
Still another object of the present invention is to provide a hydrophobic material which is inexpensive and which is applied in extremely thin layers to the ceramic probe card.
It is still another object of the present invention to provide a method of treating ceramic probe cards after construction to significantly reduce leakage currents.
It is a further object of the invention to provide a ceramic probe card with an applied hydrophobic material reducing current leakage to very low levels which is solvent resistant and stable at high temperatures.
It is yet another object of the invention to provide a hydrophobic material which can be applied to the ceramic surface and be easily removed from the conductors to avoid interfering with subsequent soldering operations.